In an optical communication system based on a wavelength multiplexed transmission, for example, an optical demultiplexer is used as a device for demultiplexing the received wavelength multiplexed light beam every wavelength into a plurality of light beams each thereof having one wavelength, and monitoring the spectrum of each of the demultiplexed light beams. The spectrum of each light beam may be detected when each light-receiving element in the array is arranged correspondingly with respect to each light beam.
An example of an optical demultiplexer will now be described with reference to FIG. 1. The optical demultiplexer comprises components such as an input optical fiber 2, a collimator lens 4, a diffraction grating 6, and a photodetector 8. These components are accommodated in three tubular members which are fitted to each other. The input optical fiber 2 consisted of a single core is fixedly coupled to a window 12 for fixing the fiber by means of a fiber coupling member 14, the window 12 being an end face of a transparent tube 10 for accommodating the fiber. The collimator lens 4 are fixed to an end of an intermediate tube 16. The diffraction grating 6 is fixed to an window 20, the window 20 being an end face of a tube 18 for accommodating the diffraction grating. In this optical demultiplexer, the tubes 10 and 18 are fitted to both ends of the intermediate tube 16 so as to be movable in the direction of light axis and rotatable around the light axis for active alignment.
The light beam from the input optical fiber 2 is guided into the optical demultiplexer. At this time, the light beam is diverged due to the numerical aperture of the fiber 2. The divergent beam reaches to the collimator lens 4 and is converged into parallel light beam by the collimator lens. The parallel light beam reaches to the diffraction grating 6 and is demodulated (or isolated) into a plurality light beams every wavelength by the wavelength dispersion characteristic of the diffraction grating 6. The demodulated light beams are converted into convergent light beams, respectively, by the collimator lens 4. These convergent light beams impinge upon the window 12 which is positioned at a focal point of the collimator lens, and are arranged in one line on the window. The photodetection for each light beam may be possible, when the photodetector 8 is secured to the window 20 in such a manner that each light beam impinges correspondingly upon each light-receiving element of the photodetector 8.
There are two types, i.e. a diffusion-type or mesa-type of light-receiving element array for the device. The structure of a diffusion-type light-receiving element array is shown in FIG. 2. An n-type InP layer 24, an i-type InGaAs layer (i.e., a light-absorbing layer) 26, and an n-type InP layer (i.e., a window layer) 28 are stacked in this order on an n-type InP substrate 22. Zn is diffused into the n-type InP layer 28 to form p-type regions 30 in order to fabricate pin-photodiodes. One pin-photodiode constructs one light-emitting element.
The diffusion is isotropic so that Zn diffusion also proceeds in a lateral direction. In this diffusion-type light-receiving element array, therefore, it is limited to decrease the interval of elements, e.g., the limit value is about 50 μm.
The structure of a mesa-type light-emitting element array is shown in FIG. 3. An n-type InP layer 24, an i-type InGaAs layer 26, and a p-type InP layer 27 are stacked in this order on an n-type InP substrate 22. Both the InGaAs layer 26 and InP layer 27 are etched away to isolate the elements in order to fabricate pin-photodiodes. One pin-photodiode constructs one light-receiving element. The mesa-type light-emitting element array has no limitation for the interval of elements, so that the array pitch of the elements may be further decreased.
In the optical communication system based on the wavelength multiplexed transmission, the expansion of spectral width and the shift of wavelength may be occurred, principally in a fiber amplifier of the optical communication system, the spread of spectral width and the shift of wavelength being referred to as noise hereinafter. When such noise is occurred, the following problem is caused. That is, when N-channel light is monitored, in which light beams L1, L2, L3, . . . each having a narrower spectral width are multiplexed as shown in FIG. 4, the strict separation between a signal and a noise in each channel is impossible for the case that light-receiving elements R1, R2, R3, . . . , RN for monitoring signals are simply arranged in one line as shown in FIG. 5.
In the case of no occurrence of noise, the light-receiving element array consisting of light-receiving elements R1, R2, R3, . . . arrayed in one line as described with reference to FIG. 5 may be used, but the alignment between the demultiplexed light beams L1, L2, L3, . . . and the light-receiving elements R1, R2, R3, . . . is very difficult.
In the case that a diffusion-type light-receiving element array is used for a photodetector, the carriers caused in one light-receiving element by light absorption are laterally diffused and are transferred to the adjacent light-emitting elements. Therefore, a current flows in the light-emitting elements to which light is not incident upon, resulting in a crosstalk. As a result, the characteristic of the light-emitting element array may be deteriorated. In the case of a mesa-type light-emitting element array, while there is no crosstalk due to the lateral diffusion of carriers like the diffusion-type one, a crosstalk may easily be occurred when a part of light impinged upon one light-receiving element is incident upon the side wall of the adjacent light-receiving element.